Coronary artery disease (CAD) affects millions of people and heart disease remains the number one cause of death worldwide. Given the importance and prevalence of this type of disease, there has been considerable interest in imaging techniques capable of diagnosing CAD. Cardiovascular magnetic resonance imaging (CMR) has emerged as an important imaging technique for evaluating CAD and other heart diseases. CMR involves the application of MRI principles optimized for use in the heart. CMR provides an image of the heart and can be used to detect abnormalities in function, blood flow, edema and the presence of myocardial infarction.
One important use of CMR in the evaluation of CAD is the identification of myocardial infarction, In recent years delayed contrast enhanced CMR techniques have enabled accurate quantification of myocardial infarction. For example, Kim et al. in U.S. Pat. No. 6,205,349 entitled “Differentiating normal living myocardial tissue, injured living myocardial tissue, and infarcted myocardial tissue in vivo using magnetic resonance imaging”, describe a technique for distinguishing between normal and infracted myocardium using contrast enhanced CMR imaging. However, small subendocardial infarcts may be difficult to detect and quantify accurately as they may be obscured by the bright signal in the blood pool.
Another important use of CMR is myocardial perfusion imaging. Over the last few years, improvements in hardware, pulse sequence development, and image reconstruction algorithms have enabled high resolution imaging of the first pass of a gadolinium based contrast agent through the myocardium. This has become a methodology utilized in adenosine stress MRI to assess myocardial perfusion. One of the major limitations of current CMR perfusion imaging techniques is the dark-rim artifact. Normally, regions of decreased perfusion are subendocardial and appear dark on CMR perfusion images. The dark rim artifact is a dark region which appears at the subendocardial border of the myocardium and can be mistaken for a true perfusion defect, causing a false positive study likely resulting in further expensive and invasive diagnostic tests such as coronary angiography. This dark rim artifact results from the inherent motion of the heart, magnetic susceptibility differences between the blood pool and myocardium, and limitations to the spatial resolution resulting from rapid imaging. As the intensity of this artifact is related to the presence of a bright blood pool signal next to a darker myocardium, attenuating the signal from the blood pool using a motion-sensitized preparation will significantly reduce this type of artifact.
Both of these examples demonstrate applications where bright signal in the blood pool can reduce the diagnostic utility of CMR for evaluation of coronary artery disease. To overcome issues of bright blood signal in cardiovascular magnetic resonance imaging, multiple techniques have been developed to suppress signal from the blood pool. Foo et al. U.S. Pat. No. 6,498,946 describe a technique consisting of a non-slice selective radiofrequency (RF) inversion pulse and slice-selective re-inversion RF pulses (so called double inversion recovery (DIR)) combined with a turbo-spin-echo readout for dark blood anatomical imaging of the heart. Another paper in the public domain describes T2-relaxation weighted imaging with dark blood employing a similar pair of inversion pulses to null the blood pool (Simonetti et. al 1996). Similarly, there is prior art for using multiple RF inversion pulses for suppressing the blood signal for imaging the walls of blood vessels and for multi-slice imaging. (Fayad, et al., U.S. Pat. No. 7,369,887)
The above prior art refer to imaging of the signal of the protons without the addition of a contrast agent. A gadolinium based contrast can be administered which shortens the T1 relaxation of the protons and results in a bright signal in inversion recovery (IR) pulse sequences. The addition of a contrast agent makes blood suppression more difficult as the shorter relaxation times put higher demands on timing parameters and result in a shorter time for washout of the blood in the imaging slice. This results in incomplete suppression of the blood pool signal and causes image artifacts. Foo et al. (U.S. Pat. Nos. 6,662,037 and 6,526,307) describe a technique for nulling the signal from the blood pool by combining a “notched rf pulse” which effectively suppresses the blood signal outside of the slice of interest, and with blood flow in the heart this suppressed signal moves into the slice of interest and is rendered dark. This technique is susceptible to errors in the slice profile of the “notched” rf pulse as well as requiring all of the blood to move out of the slice to null the signal.
Two other techniques for suppressing the blood pool in contrast enhanced imaging of myocardial infarction have been described. Rehwald et al. (U.S. Patent Application Publication No. US 2009/0005673 A1 (Ser. No. 11/957,520)) have developed a technique based on the combination of a slice selective rf pulse and a non-selective rf pulse with precise timing which nulls the signal from the blood pool. While this technique greatly improves contrast between the blood pool and the infarct, it does so at an expense of signal-to-noise ratio and contrast-to-noise with respect to the normal myocardium. The technique also has some susceptibility to slow flowing blood and changes in the parameters as a function of the magnetic relaxation parameters of the heart. A pulse sequence by Ibrahim et al., which is based on the stimulated echo acquisition-mode (STEAM) technique, also has the ability to suppress the signal from the blood pool (Ibrahim et. al. 2008). However, this technique requires three separate images of the heart, and suffers from STEAM's inherent 50% decrease in SNR.
A different method for suppressing the signal from the blood relies on phase dispersion related to the inherent self-diffusion coefficient of water. This idea was first described for suppressing the blood signal for imaging of the brain with a so called “arterial-spin labeling” technique (Pell et al. 2003). This concept was extended to contrast enhanced vessel wall imaging (Koktzoglou et. al. 2007). In this technique a diffusion prepared driven-equilibrium fourier transform (DEFT) preparation is used consisting of a 90 degree rf pulse followed by a strong magnetic field gradient, then followed by a 180 degree pulse another magnetic field gradient and finally a negative 90 degree pulse to null the blood signal based on the high self diffusion coefficient of water (2.2×10−3 mm2/s). In both of these applications large gradients are played out resulting in diffusion attenuation coefficients (b-values) of 0.7 s/mm2. Very recently, this concept has been extended to non-contrast imaging of the heart combining a DEFT preparation with a steady-state free procession readout scheme (Nguyen et al. 2008).
However, none of the current techniques apply motion-sensitized dark-blood techniques for imaging of first-pass perfusion or delayed enhancement of the myocardium with gadolinium-based contrast agents.